Electron paramagnetic resonance of switchable copper-nitroxide-based molecular magnets: An indispensable tool for intriguing systems

Copper-nitroxide-based molecular magnets of the Cu(hfac)2LR family (often called “breathing crystals”) are interesting and appealing systems exhibiting various magnetic anomalies triggered by temperature or light, which can generally be viewed as spin-crossover-like phenomena. The presence of paramagnetic copper(II) ions and stable nitroxide radicals in the polymer chains of breathing crystals makes electron paramagnetic resonance (EPR) widely applicable to these systems. In addition to the determination of typical parameters (g-tensor, hyperfine splitting constants, etc.), EPR allows the monitoring of magnetostructural switching between different states and evaluation of exchange interactions in the exchange-coupled clusters of copper(II) with nitroxides and those between the exchange-coupled clusters. This review highlights the capabilities of EPR in investigating breathing crystals and demonstrates the obtaining of crucial information on magnetic interactions inaccessible by other techniques.